This invention relates to phase locking diode laser arrays, and more particularly to a technique for phase locking the outputs of the diode lasers in the array with inherent light from a master diode laser.
Diode lasers do not emit light with sufficient power for most applications, such as communications. Consequently, the practice has been to provide an array of edge emitting stripe lasers, typically 5 .mu.m wide on 9 .mu.m centers. These lasers are coupled for coherent operation, usually by techniques designed into the array structure. However, these edge emitting diode lasers are, at present, inherently one dimensional, with the lasing occurring parallel to the substrate. To increase the output power, it would be desirable to stack several of these linear arrays; the problem is to provide a way of causing them to operate in phase. Much work has been devoted to the analysis, development and fabrication of diode laser arrays. D. R. Scifres, R. D. Burnham and W. Streifer, "Phase-Locked Semiconductor Laser Array," Appl. Phys. Lett. Vol. 33, pp. 1015-1017, 1978; D. E. Ackley, "High Power Multiple Stripe Injection Lasers with Channel Guides," IEEE J. Quantum Electron., Vol. QE-18, pp. 1910-1917, 1982; E. Kapon, C. Lindsey, J. Katz, S. Margalit and A. Yariv, "Coupling Mechanism of Gain-Guided Integrated Semiconductor Laser Arrays," Appl. Phys. Lett. 44, Feb. 15, 1984.
A desirable property of such arrays is to have the output beams from each of the laser stripes phase coherent (relative to one another). Sometimes this phase coherence can be produced by simply placing the laser stripes close enough together. E. Kapon, J. Katz, C. Lindsey, S. Margalit and A. Yariv, "Control of Mutual Phase Locking of Monolithical Integrated Semiconductor Lasers," Appl. Phys. Lett. 43, pp. 421-42 (1983); J. Katz, E. Kapon, C. Lindsey, S. Margalit, U. Shreter and A. Yariv, "Phase Locked Semiconductor Laser Array with Separate Contacts," Appl. Phys. Lett. 43 pp. 521-523, (1983). However, such coupling can often lead to undesirable supermode patterns in the array far field. E. Kapon, J. Katz, and A. Yariv, "Supermode Analysis of Phase Locked Arrays of Semiconductor Lasers," Opt. Lett., Vol. 9., pp. 125-127, 1984.
A more positive method to achieve phase locking is via injection locking where a master laser is focused into the back facet region of the array, thereby injecting a phase coherent stimulus into all the array channels. Some success has been achieved experimentally using this technique. L. Goldberg, H. F. Taylor and J. F. Weller, "Optical Injection Locking of a GaAlAs Phase Coupled Laser Array," presented at XIII International Conference on Quantum Electronics, Postdeadline paper PD-B4, Anaheim, CA, June 12, 1984; J. Daher, "Injection Locking of an Incoherent Laser Array," Summer Internal Summary Report, Jet Propulsion Laboratory, August 1984. Since the array stripes (channels) are quite close together (usually about 9 .mu.m spacing for a 5 .mu.m channel), coupling of the master laser power into the array channels can be accomplished with reasonably high geometric efficiency using a cylindrical lens.
One of the major drawbacks of present laser diode arrays formed in a body of semiconductor material is that they are inherently one dimensional, as noted above. The laser channels are defined by laying down contact stripes on top of thinly grown layers deposited on a substrate. The channels are defined by contact stripes, or other means, such as index guiding of the lasing which occurs parallel to the substrate surface with outputs occurring at the edges of the sheet of grown layers. It is for that reason that these devices have been restricted to one-dimensional arrays.
One solution to the problem of providing more laser diodes in an array is to make a two-dimensional, surface-emitting (SE) array. Both GaAlAs/GaAs and GaInAsP/InP systems have been used to fabricate a surface-emitting injection laser. See A. Ibaraki, S. Ishikawa, S. Ohkouchi and K. Iga, "GaAlAs/GaAs Surface Emitting Injection Laser," Proceedings, Thirteenth International Quantum Electronic Conference, Anahiem, CA, June 18, 1984, paper MLL7, pages 49-50, S. Uchiyama and K. Iga, "Two Dimensional Array of Improved Gain AsP/InP Surface-Emitting Injection Lasers" and Proceedings, 1985 Conference on Lasers and Electro-optics, paper No. TUF6, page 45. In these devices, layers are first grown on a substrate. Then etched laser facets (pothole mirrors) are etched from the bottom (through the substrate) and laser facets are etched or otherwise provided on the top. In such devices, the lasing output occurs normal to the substrate surface.
Although such devices hold promise for two-dimensional structures, the separation between laser front facets (potholes) is usually 25 to 100 .mu.m. This separation, along with the short laser cavity length, effectively prevents any phase coupling between the lasers. Thus, the only method by which this type of laser array could be phase locked is through injection locking. However, the geometric efficiency (i.e., aggregate area of the back facets divided by the area of the array) is extremely small. This is true to a lesser extent even of a linear array, so that it is difficult to use injection locking there as well because in both cases the geometry makes coupling a master laser into every diode of the array virtually impossible. So the problem is to phase synchronize the outputs of the diode lasers in the array such that the combined output is phase coherent and contains negligible far-field supermodes, i.e., produces a single lobe far-field pattern, using a master diode laser.